The invention relates to a valve assembly for actuation of a user, and a method for actuating such a valve assembly.
The like valve assemblies are employed in mobile hydraulics for actuating single and double-action users such as, for example, hydraulic motors or power lifters. Herein a throttle device whereby the supplied or drained volumetric flow of hydraulic oil may be throttled is arranged in each of the work conduits leading to the user, i.e., in the supply conduit and in the drain conduit.
In FIGS. 1 to 3, which shall even now be referred to, several exemplary applications of such known throttle devices are represented.
FIG. 1 shows a valve assembly for actuating a double-action user which may, for example, be a lifting cylinder 200. To the cylinder chamber and to the annular chamber of the lifting cylinder 200 a work conduit 206 or 208 is connected. For outward movement of the lifting cylinder 200, the hydraulic oil is supplied via work conduit 206 to the lifting cylinder 200, with the hydraulic oil displaced from the annular chamber being returned to the tank through the work conduit 208 which acts as a drain conduit.
For the purpose of throttling the hydraulic oil supply and drain, two valve throttles 210, 212 are arranged in the two work conduits 206 and 208, the valve members of which are coupled mechanically. The ratio of supply and drain opening of the valve throttles is determined by the mechanical coupling of the opening lands of the common spool, so that at a particular setting of the valve throttle 210, a pressure loss .DELTA.p2 will ensue in the drain conduit (work conduit 208). This pressure loss is undesirable in particular in where single-action users are connected as it does not fulfil a meaningful function. The pressure loss results in energy losses, in heating of the hydraulic oil, and eventually in premature wear of the valve throttle.
In FIG. 2 a valve assembly for actuating a blower motor 214 is represented. In this alternative, two independently apportionable valve throttles 210, 212 are used which may each be employed in a motor having two directions of rotation for throttling of the volumetric flow of hydraulic oil in the supply conduit. In a case where a motor having one direction of rotation is used, drain throttling--in this case the valve throttle 212 in the drain conduit 208--causes unnecessary pressure losses, so that the above specified drawbacks also exist in this type of valve assembly.
FIG. 3 shows an application wherein a lifting cylinder 200 is used for lifting or lowering an extension arm 215 of lifting gear 216. In order to lift the load m, the cylinder chamber of the lifting cylinder 200 is supplied with hydraulic oil, so that the cylinder moves outward and the extension arm 215 is moved upward in the representation of FIG. 3. The hydraulic oil present in the annular chamber is displaced as a result of the outward movement and returned to the tank. As a result of the load m exerting a tensile force on the extension arm 215, a resulting force acts on the lifting cylinder 200 which is maximum when the extension arm 215 extends horizontally and diminishes concurrently with a rotation of the extension arm 215 from this horizontal position. The further the extension arm 215 is lowered into the horizontal position, the higher becomes the weight on the drain side of the lifting cylinder 200, so that the latter is pressed downward in the representation of FIG. 3. Due to the rising volumetric drain flow Q2, cavitation may occur in the supply conduit which continues to receive a volumetric supply flow Q1, whereby the components of the valve assembly are strained in a maximum degree, which may also bring about control instability. For this reason it is also necessary to adapt drain throttling to the lifting conditions of the lifting gear. Under particular operating conditions, such drain throttling generates pressure losses which are not acceptable for the above named reasons.
In order to avoid such unnecessary pressure losses, so-called "non-pressurised tank returns" have, for example, been provided in motor applications for farming tractors, which may be connected to the drain side of the motor by means of a quick coupling and whereby the drain throttle may be bypassed. It is a drawback in such an auxiliary construction that the vehicle must be provided with a separate tank connection piece including a quick coupling, and that the operator must manually connect the non-pressurised tank return. Such an auxiliary construction can be sensibly employed only if it is intended to maintain the respective operating condition over a prolonged period of time. In the case of a change of user actuation, for example a reversal of direction of rotation of the motor etc., ports must be reconnected, so that the requirements in terms of operation are considerably high.